Donald Trump vs. Jerry Seinfeld: Battle of the NBC All-Stars, Round 2

Hot off his feud with Bill Cosby, Donald Trump is taking on another NBC comedy icon: The Donald is furious at Jerry Seinfeld for canceling an appearance for his son's foundation.

Seinfeld canceled on the Sept. 13 event benefit for the St. Jude Children's Research Hospital because he disagrees with Trump questioning President Obama's citizenship, his reps told to the New York Post.

Trump, in turn, fired of a letter to the "Seinfeld" star criticizng him and Seinfeld's "The Marriage Ref."

"I just learned you canceled a show for my son's charity," Trump wrote, according to the Post, "because of the fact that I am being very aggressive with respect to President Obama, who is doing an absolutely terrible job as our leader."

"We don't care that you broke your commitment," he continued, "even though the children of St. Jude are very disappointed, and despite the fact that your manager clearly stated you are 'truly a man of his word.'

"What I do feel badly about is that I agreed to do, and did, your failed show, 'The Marriage Ref,' even though I thought it was absolutely terrible . . . Despite its poor ratings, I didn't cancel on you like you canceled on my son and St. Jude. I only wish I did."

Trump signs off, "You should be ashamed of yourself!"

"Failure" is subjective, but NBC's "The Marriage Ref" hasn't been canceled: It returns June 26.

Seinfeld agreed to perform at the event in January, before Trump began to discuss a presidential run. But he has since "grown increasingly uncomfortable" with Trump's questioning Obama's citizenship, a Seinfeld rep told the Post. Seinfeld will instead make a contribution to the Eric Trump Foundation and St. Jude.

"Celebrity Apprentice" winner Bret Michaels will take over Seinfeld's slot.